Beneficial nematodes are microscopic roundworms that are deadly to insect larvae but absolutely safe to humans, pets, and plants. They kill the flea larvae in your yard by ambushing them, boring inside of them, and then releasing a bacterium which kills and partially digests the insect thus providing food for itself. (I’m sure glad these things don’t attack humans, this would be a horrible, although unique, way to die.)

Interestingly, nematodes are the only effective way to attack fleas during their pupae (cocoon) stage. This means that they have the potential to significantly destroy much of your yard's flea population before they become fleas. This is important because you'll be interrupting the life cycle of the flea and preventing the next generation from infesting your yard.

Nematodes won’t do well in a hot, dry lawn. This means that you’ll either need to keep your entire lawn watered or you’ll have to concentrate on the areas your pet, and therefore the fleas, are most likely to be. For example, we have a pretty good sized yard and here in South Dakota the summers are hot and dry. It’s all we can do to keep the grass alive and watering is mostly an exercise in futility. However, we do have a ton of trees too and the shaded grass is a bit cooler, a little greener, a little more damp, and that is where our dogs are most likely to hang out. This means that I can apply nematodes under the trees where the dogs are and essentially ignore the rest of the yard.

The two exceptions to this are in the late spring and early fall. It is generally a bit wetter and a bit cooler thus making living conditions a bit better for the fleas. Given the fact that stray cats are constantly wandering through our property, it is only prudent to give the entire yard a dose of nematodes at these times of year to interrupt any breeding plans they might have.

Nematodes & Yard Chemicals

According to a website maintained by Ohio State University’s Department of Entomology:

“Fertilizers should be avoided roughly 2 weeks prior to and after nematode application, because they may be adversely affected by high nitrogen content.”

The site continues…

“Some pesticides work well with nematodes when their mutual exposure is limited while other pesticides may kill nematodes. Check labels or specific fact sheets to find out. Some chemicals to avoid are bendiocarb, chlorpyrifos, ethoprop, and isazophos. Fungicides to avoid are anilazine, dimethyl benzyl, ammonium chloride, fenarimol, and mercurous chloride. The herbicides, 2,4-D and trichlopyr and nematicide, fenamiphos, should be avoided as well.”

What this means is that generally speaking you don’t want to use nematodes in conjunction with other lawn products unless you really know what you are doing.

Which species?

The species you want is Steinernema carpocapsae (the label might just say S. carpocapsae). These are “ambushers” which will attack any flea larvae that might happen by and thus prevent them from becoming adults. They are also the only way to kill fleas while still in their cocoon (pupal) stage. They will also attack other grub-like, soil dwelling creatures but don’t worry, they won’t go after any of the “good” ones and they leave earthworms alone.

Try to avoid buying mixed species of nematodes. They will kill other bugs, which is fine if that is what you want, but what happens is that you end up diluting your flea control efforts. Instead of 100% of your nematodes hunting for flea larvae, only 50% of them will be. It's not a big deal, just keep it in mind.

Application

Nematodes come on a sponge which you soak in up to maybe a gallon of water (each company is different). Once you are satisfied that you've gotten most of them off the sponge, you simply use this water to spray down the desired area. Remember, they don't like heat or sunshine so a cool, slightly rainy day would be perfect. If that isn't possible, apply them in the evening and be sure to pre-water the area and to water it again the next day. You do not want their environment to dry out.

Beneficial nematodes will work best in sandy soil, bare ground, and in areas with minimal vegetation. They are reported to work great in kennels with cement floors. The ground must be kept wet so a quick, daily spraying might be of use (it depends on your yard).

You'll need to re-apply nematodes every 4 to 6 weeks for them to be the most effective. If I was feeling cheap, I think I'd do one batch in early spring and then another in late summer/early fall. That would probably be good enough.

Although they are kind of expensive, they are far safer than pesticides and they are a great way to round out your flea control efforts.

Flea Wars